The present invention relates to a servo-drive device for the positioning of an actuating element, the setting range of which comprises a plurality of stable setting positions, and more particularly, to a servo-drive device having a servo motor configured to drive the actuating element and a control device configured to control the servo motor and to identify actuating-element position with motor current.
Servo-drive systems serve, for example, to adjust and position ventilation dampers in a motor vehicle or other final control elements of closed-loop or open-loop control circuits. Conventional stepping motors with gearing, d.c. motors with a flange-mounted potentiometer at the gear output, d.c. geared motors with Hall sensors integrated in the motor and pneumatic devices provide the drive. In order to prevent unintentional adjustment of the actuating element by external disturbances, in the case, for example, of a ventilation damper by the effect of the air stream, it is customary in servo drives with a gear mechanism to use a self-locking gear mechanism. With a stepping motor, the positioning of the actuating element is performed by step-by-step driving and counting of the triggering pulses; in the case of a d.c. motor provided with a potentiometer, the positioning is performed by measuring the potential voltage proportional to the angle of adjustment, in the case of a d.c. motor provided with a Hall sensor, the positioning is performed by counting the Hall pick-up pulses and in the case of pneumatic actuating elements, the positioning is performed, for example, by a flange-mounted potentiometer.
If the position detection is performed by a position sensor system, a separate electrical supply line is needed which, for example, in a motor-vehicle air-conditioning installation with about 10 damper-adjusting units represents a considerable additional outlay. If a simple d.c. motor is used, the outlay for the position check-back signal is consequently about as great as for the actual drive motor. With pneumatic actuating elements, a pressure-supply device for generating above-atmospheric and below-atmospheric pressure and also suitable solenoid valves for individual triggering of the final control elements are required.
DE 38 35 773 C2 shows a servo drive for an air-conditioning installation in which, for positioning, contact brushes which lie in the motor circuit and move at the same time when there is an adjusting movement are arranged such that they can be electrically connected via an opposing, fixed printed conductor pattern. To be able to bring a damper, located in the air stream, into various setting positions with this servo-drive, the printed conductor pattern is connected to a corresponding number of user-operable setting switches and is designed such that, when a switch is operated, the motor circuit between the two brushes remains closed via a printed conductor pattern until, due to the turning motion, the brushes assume a position in relation to the printed conductor pattern in which the motor circuit is interrupted. This position then corresponds to the desired damper position, in which the damper subsequently remains, on account of the interrupted motor circuit, until some other requirement arises by the user operating the switch.
It is known, furthermore, from, by way of example, DE 33 46 366 A1, DE 35 14 223 A1 and DE 39 33 266 A1 to detect the current intensity in the circuit of the servo motor and to drive the servo motor in a way dependent on the current intensity. Based on this principle are, for example, arrangements which, if an obstacle is detected in the travel of the actuating element, switch off the servo motor and/or operate it in the reverse direction, for example, as a protection against damage to the actuating element or as a protection against jamming.
Furthermore, DE 39 35 593 A1 describes a device for controlling the temperature in a motor vehicle interior in which, to be able to dispense with the expensive potentiometer for position identification, a d.c. motor is driven under clock control and the position identification is performed by evaluating the detected motor current in that a motor current edge identification is carried out for each further running of a motor armature segment. These identified edges are counted, and this counted value serves the controller as a measure of the position of the final control element. Apart from the pulse-counting device required, in the case of this servo-drive system suppression of the natural running-on of the d.c. motor is provided.
DE 38 32 474 A1 describes a servo drive with which a dividing wall for optionally closing one or two intake paths of an air-conditioning unit can be positioned by a simple electric motor, which can be driven in one rotational direction, and a counteracting spring element optionally in one of the two end positions of the setting range, defined by end stops, without the possibility of setting a stable intermediate position. For this purpose, the motor and the spring element are matched to each other so that the torque generated by the motor is greater than the maximum torque which can be generated by the spring restoring force. When the servo motor is switched on, the dividing wall consequently assumes the end position corresponding to its direction of torque. From that end position, the dividing wall then can be moved into the other end position and held there by simple switching off of the motor, on account of the restoring action of the spring element.
An object of the present invention is to provide a servo-drive device which can be realized with comparatively little outlay and permits a positioning of the actuating element in each of the plurality of envisaged setting positions which is exact and stable with respect to external effects.
This object has been achieved in accordance with the present invention by a servo-drive device by a mechanical system moved at the same time as adjusting movement of the actuating element, having potential energy varying along its path of movement, wherein motor current characteristic over an actuating-element setting range is determined by force exerted by the mechanical system on the actuating element.
The variation of the potential energy of the mechanical system over the setting range of the actuating element results in corresponding fluctuations of the force exerted by the mechanical system on the actuating element and consequently of the current intensity in the circuit of the servo motor during an adjusting movement. The motor is, of course, sized and configured such that the torque which can be generated by it is greater than the maximum counteracting moment exerted by the mechanical system. The control device is consequently able to identify the respective position of the actuating element by the motor current detection on the basis of the motor current fluctuations effected by the mechanical system and specific to the position of the actuating element, without requiring a dedicated position sensor system for the actuating element or for the motor, with corresponding signal lines to the control device.
According to another aspect of the present invention, positions of local minima of the potential energy of the coupled mechanical system define stable setting positions envisaged for the actuating element. A consequent major advantage is that the exact positioning of the actuating element by the mechanical system can be ensured and therefore no correspondingly expensive control for the servo motor is necessary. The servo motor only has to be chosen such that in operation it overcomes the forces acting on the actuating element from the mechanical system. As soon as the control device establishes, by evaluation of the motor current variation, that the actuating element is located in the drawing-in region of the local minimum of the potential energy of the mechanical system associated with the desired setting position, the exact positioning can be performed in a simple way by switching off the motor, under the action of the mechanical system alone. Then, the mechanical system moves automatically into this local minimum of the potential energy which, in turn, corresponds exactly to the desired position of the actuating element. Therefore, precision timing of the switching off of the motor current is not required for the exact positioning of the actuating element. For the same reason, the customary measure of electrically short-circuiting the motor when stopping it, for the purpose of preventing run-on, is also not required. Furthermore, the assignment of a local minimum of the potential energy of the mechanical system to each stable actuating-element setting position has the effect that the actuating element is held stably with respect to external effects in the respectively desired position by the restoring forces of the mechanical system, without for example a self-locking gear mechanism being required for this purpose.
Another feature of the present invention is that the motor is in each case switched off by the control device after reaching the drawing-in region of the required position of the actuating element formed by the mechanical system, as soon as an increase in motor current is established there. On account of the restoring forces of the mechanical system occurring upon leaving the energy minimum, in each case a usually considerable and consequently easy-to-detect increase in motor current occurs in the vicinity of a stable actuating element position.
Alternatively, the motor circuit is interrupted by the mechanically operable switching element in the vicinity of each stable actuating element position. This motor circuit interruption is replayed by the control device, by way of suitable closing of the parallel switching element, in each case until the system is again outside this vicinity or the drawing-in region of the desired setting position is reached.